1. Field of the Invention
The present invention relates to a display device having a pixel array in which pixel circuits using an organic electroluminescence element (organic EL element) or a liquid crystal element are arranged in the form of a matrix, and a display method.
2. Description of the Related Art
An active matrix type display device using for example an organic electroluminescence (EL) light emitting element in pixels is known. This display device controls a current flowing through the light emitting element inside each pixel circuit by an active element (typically a thin film transistor: TFT) provided inside the pixel circuit. That is, because the organic EL element is a current light emitting element, an amount of current flowing through the EL element is controlled to obtain a coloring gradation.
[Patent Document 1]
Japanese Patent Laid-Open No. 2003-255856
[Patent Document 2]
Japanese Patent Laid-Open No. 2003-271095
FIG. 35 shows an example of an existing pixel circuit using an organic EL element.
Incidentally, though only one pixel circuit is shown in FIG. 35, an actual display device has pixel circuits as shown in the figure arranged in the form of an m×n matrix, and each pixel circuit is selected and driven by a horizontal selector 101 and a write scanner 102.
This pixel circuit has a sampling transistor Ts formed by an n-channel TFT, a storage capacitor Cs, a driving transistor Td formed by a p-channel TFT, and an organic EL element 1. The pixel circuit is disposed at a part of intersection of a signal line DTL and a writing control line WSL. The signal line DTL is connected to one end of the sampling transistor Ts. The writing control line WSL is connected to the gate of the sampling transistor Ts.
The driving transistor Td and the organic EL element 1 are connected in series with each other between a power supply potential Vcc and a ground potential. The sampling transistor Ts and the storage capacitor Cs are connected to the gate of the driving transistor Td. The gate-to-source voltage of the driving transistor Td is denoted by Vgs.
In this pixel circuit, when the writing control line WSL is set in a selected state, and a signal value corresponding to a luminance signal is applied to the signal line DTL, the sampling transistor Ts conducts to write the signal value to the storage capacitor Cs. The potential of the signal value written to the storage capacitor Cs becomes the gate potential of the driving transistor Td.
When the writing control line WSL is set in a non-selected state, the signal line DTL and the driving transistor Td are electrically disconnected from each other. However, the gate potential of the driving transistor Td is retained stably by the storage capacitor Cs. Then a driving current Ids flows through the driving transistor Td and the organic EL element 1 in a direction from the power supply potential Vcc to the ground potential.
The current Ids at this time is a value corresponding to the gate-to-source voltage Vgs of the driving transistor Td. The organic EL element 1 emits light at a luminance corresponding to the value of the current.
That is, in the case of this pixel circuit, the voltage applied to the gate of the driving transistor Td is changed by writing the potential of the signal value from the signal line DTL to the storage capacitor Cs. The value of the current flowing through the organic EL element 1 is thereby controlled to obtain a coloring gradation.
The source of the driving transistor Td formed by a p-channel TFT is connected to a power supply Vcc, and the driving transistor Td is designed to operate in a saturation region at all times. The driving transistor Td is therefore a constant-current source having a value shown in the following (Equation 1).
                    Ids        =                              1            2                    ⁢          μ          ⁢                                          ⁢                      W            L                    ⁢                                    Cox              ⁡                              (                                  Vgs                  -                  Vth                                )                                      2                                              [                  Equation          ⁢                                          ⁢          1                ]            
where Ids denotes the current flowing between the drain and source of the transistor operating in the saturation region, μ denotes mobility, W denotes a channel width, L denotes a channel length, Cox denotes a gate capacitance, and Vth denotes the threshold voltage of the driving transistor Td.
As is clear from this (Equation 1), the drain current Ids of the transistor in the saturation region is controlled by the gate-to-source voltage Vgs. Because the gate-to-source voltage Vgs of the driving transistor Td is retained at a constant level, the driving transistor Td operates as a constant-current source to be able to make the organic EL element 1 emit light at a constant luminance.